The present disclosure relates to an improved efficient and effective method of manufacturing hydrophilic polyethersulfone (PES) membrane suitable for commercial applications and the resultant hydrophilic polyethersulfone (PES) membrane suitable for commercial applications produced thereby.
As is known, PES membranes are naturally hydrophobic. Most membrane applications require the use of hydrophilic membranes. Several different methods are known to transform hydrophobic PES membranes into hydrophilic PES membranes (to perform membrane hydrophilization). Some of these methods are complicated and expensive, while others fail to provide high purity membrane (for example, the membranes could contain the remains of hazardous monomers, used for hydrophilic coating).
Several different prior known methods of PES membrane hydrophilization are presented in the patent and scientific literature. In one known prior method, the hydrophilization of PES membrane was accomplished by coating the hydrophobic membrane with a hydrophilic polymer. In order to provide the desirable permanent attachment of the hydrophilic polymer to the membrane, a hydrophilic coating layer was usually subjected to a cross-linking reaction or a coating polymer was grafted to the surface of the hydrophobic PES membrane. The preceding approach has been disclosed in the following patents and publications:
U.S. Pat. No. 4,618,533 disclosed a method of PES membrane hydrophilization by direct membrane coating. As described, the hydrophobic membrane was prewetted with alcohol, and then soaked in aqueous solution that contained a hydrophilic monomer, a polyfunctional monomer (cross-linker) and an initiator of polymerization. The monomer and cross-linker were then polymerized using thermal or UV initiated polymerization, which formed a coating of cross-linked hydrophilic polymer on the membrane surface.
U.S. Pat. Nos. 6,193,077 B1 and 6,495,050 B2 proposed coating the PES membrane by soaking the membrane in an aqueous solution of hydrophilic polymer (polyalkylene oxide) and at least one polyfunctional monomer (cross-linker), then polymerizing a monomer. As described, a non-extractable hydrophilic coating was the resultant.
The article “Surface modification of Poly(ether sulfone) Ultrafiltraton Membranes by Low-Temperature Plasma-Induced Graft Polymerization (Journal of Applied Polymer Science, Vol. 72, 1699-1711 (1999)) describes the hydrophilization of PES membrane by a grafting reaction. In this process as described therein, hydrophilic PES membrane was submitted to low-temperature helium plasma treatment followed by the grafting of hydrophilic monomer N vinyl-2-pyrrolidone onto the membrane surface.
In another known prior method, the hydrophilization of PES membrane was accomplished by dissolving hydrophobic PES polymer in a solvent and blending it with a hydrophilic additive, which was soluble in the same solvent. The obtained blended solution was used for casting a hydrophilic membrane.
The following patents disclose representative prior methods of PES membrane hydrophilization by blending PES polymer with hydrophilic additives.
U.S. Pat. No. 4,943,374 proposed to blend PES in a solution with hydrophilic polymers (polyethylene glycol, PVA, polyacrylic acid, polyvinilpirrolidone, etc.). According to the patent, the resultant membranes obtained from the blended solutions were hydrophilic.
U.S. Pat. No. 6,071,406 disclosed the production of hydrophilic PES membranes by blending PES in a solution with a wetting agent (a block copolymer having hydrophilic and hydrophobic units). In the resultant membrane, the hydrophobic units of the block copolymer were permanently attached to the hydrophobic matrix (PES) leaving the hydrophilic units on the membrane surface. Since, according to this patent, the wetting agent was permanently attached to the membrane and could not be leached, the resultant membrane possessed permanent hydrophilicity.
U.S. Pat. No. 5,178,765 disclosed the hydrophilization of PES membrane by blending PES with hydrophilic poly-2-oxazoline resin and polyvinylpyrrolidone resin. According to this patent, the membrane obtained thereby exhibited long-term water wettability.
U.S. Pat. No. 6,495,043 B1 disclosed a method for PES membrane hydrophilization by blending PES with hydrophilic ethylene oxide/propylene oxide copolymer. According to this patent, the resultant hydrophilic membrane had a reduced tendency toward fouling. As is known to those skilled in the art, the term “fouling” means clogging the membrane pores during the filtration process.
U.S. Pat. No. 6,039,872 disclosed a method of producing hydrophilic PES membrane by blending the PES with a hydrophilic monomer and an initiator for thermal polymerization. After blending, the polymer solution was heated to a temperature sufficient to start a polymerization of the blended monomer. The resultant polymer solution reportedly contained a blend of PES with hydrophilic polymer. The membrane produced from this resultant solution was reportedly hydrophilic.
U.S. Pat. No. 4,964,990 disclosed a method, which included a combination of blending PES in a solution with a hydrophilic additive, followed by a hydrophilic coating of the membrane. In the method described in this patent, the PES was mixed in a solution with a hydrophilic polymer (polyethylene glycol or polyvinylpyrrolidone), and then the membrane was cast, quenched and dried. The dried membrane was post treated with an aqueous solution of polyvinyl alcohol and then cross-linked. The patent claimed that the resultant membrane possessed permanent wettability and stability after exposure to prolonged treatment in isopropanol or extended heat treatment.
In yet another known prior method, the hydrophilization of PES membrane was accomplished by treatment with low temperature plasmas. The following publications describe the application of plasma reactions for PES membrane hydrophilization:
The dissertation “Surface modification of porous polymeric materials using low-temperature plasmas” (Michelle L. Steen, Colorado State University, 1994) described a surface modification of several membranes from different polymers, including PES. To impart permanent hydrophilic properties to these membranes, the membranes were treated with low-temperature plasma. It was reported that the plasma treatment initiated formation of hydroxyl radicals (OH radicals). OH radicals were the primary reactive species involved in membrane modification. Because of the influence of OH radicals, the oxidation reaction occurred, and hydrophilic groups containing oxygen appeared on the membrane surface. It was reported that the presence of these polar groups made the membrane hydrophilic.
The article “Modification of porous Poly(ether sulfone) Membranes by Low-Temperature CO2 Plasma Treatment” (Journal of Polymer Physics, Vol. 40, 2473-2488 (2002)) described the hydrophilic modification of PES membrane by treatment with low temperature CO2-plasma. The article claims the formation of hydrophilic functionalities on the membrane primarily during a plasma treatment, with some incorporation of atmospheric oxygen and nitrogen on the membrane surface immediately upon exposure the membrane to air.
Shortcomings of the above described prior methods are presented below.